The invention relates to a method for the reproduction of a symbol with an adjustable scale and/or an adjustable position on an imaging surface by means of successions of brightness amplitudes which are arranged in the image lines of an image raster on the imaging surface, one of at least three values being used for each brightness amplitude, each brightness amplitude corresponding to a low-pass spatially filtered amplitude of an ideal brightness profile of the symbol, sampled on a sampling grid comprising sampling lines with a pitch which is controlled relative to the ideal brightness profile by the adjusted scale and/or with an offset which is controlled relative to the ideal brightness profile by the adjusted position with an accuracy amounting to a fraction of the pitch. The invention also relates to a device for performing such a method.
A method of the kind set forth is known from A. Naiman, A. Fournier, "Rectangular convolution for fast filtering of characters", Computer Graphics, Vol. 21, No. 4 (July 1987), pp. 233-242. According to the cited method, the brightness profile expressed in a symbol description with high-resolution information is converted into successions of brightness amplitudes, each of which is arranged in a line on an image raster. When use is made of, for example a page description language such as POSTSCRIPT (R) (a trade mark owned by ADOBE), which is described in the "Postscript language reference manual", Adobe systems, Addison Wesley publishing company, Reading Massachusetts, 1985, ISBN 0-201-10174-2, the ideal brightness profile is described by a number of mathematical curves which represent, for example the edge between "black" and "white" in a letter symbol. The curves themselves are defined by parameters. This kind of description has an infinitely high resolution; this is what is meant by ideal. The ideal brightness profile can alternatively be described by way of a master grid of brightness amplitudes, the brightness profile on the master grid then being ideal in a sense that its resolution is higher than that of the sampling grid. Generally speaking, an ideal brightness profile is to be understood to mean any brightness representation which contains more detailed information than the ultimate succession of brightness amplitudes.
When a page description language is used, moreover, the position and scale of symbols to be reproduced can be highly accurately indicated. Thus, a symbol to be reproduced will be specified independent of the image display device, and one does not need to take its properties into account in the specification. The ideal symbols defined in the page description language cannot be reproduced exactly on simple image display devices, which, of course, have only limited resolution. However, according to the known symbol reproduction method, the resolution of preceived details can be improved in known devices by utilizing more than two brightness amplitude values, even though the brightness profiles themselves are actually bivalent, for example typographic symbols such as letters, digits etc. which have a foreground brightness value for "inked" segments and a background value against which these segments are reproduced ("black" and "white", respectively in printed text).
In order to improve the perceived resolution, the known method utilizes the known fact that the human visual system does not interpret brightness values between the foreground value and the background value in otherwise bivalent patterns as brightness values per se but rather as high-resolution details. Thus, an image line having a width of one raster line and a brightness value halfway between the foreground value and the background value is interpreted as a line having a width of one half pixel against the background. A stepped brightness profile comprising image lines having background values to one side of the step, image lines having foreground values to the other side of the step, and at the edge therebetween an image line having a brightness value halfway the foreground value and the background value is interpreted as a stepped profile halfway the image line therebetween.
Although the cited article by Naiman and Fournier, restricts the method to the use of grey values as brightness amplitudes, it will be evident that this restriction is not essential; for example, colour amplitudes and combinations thereof can also be treated in this manner. The devices in which the described method is carried out need not be restricted to CRTs either; the method can be used in any device capable of reproducing images with more than two brightness amplitudes, for example LCDs and printers.
It is a drawback of the known method that the prevention of artifacts, i.e. perceived deviations from the desired brightness profile, necessitates a complex operation so as to extract the succession of brightness amplitudes from the brightness profile of a symbol. This is especially disadvantageous since it is necessary to take into account properties of the display panel; for example, in the case of a CRT it is necessary to take into account the linearity of the phosphor and the shape of the pixels which differ from one type of semen to another and sometimes even from one screen to another. Therefore, usually artifacts remain: the symbols are perceived as being unsharp, the baseline on which typographic symbols rest is perceived to be undulating, and so is the top line (x-height) extending along the tops of the typographic symbols. As a result, the reading of the screen is more fatiguing than the reading of conventional printed matter.